This invention relates to controlled current sources of two-wire measuring instruments which generate a measurement signal in the form of an output current between 4 mA and 20 mA and which are controlled by a control signal derived from an output signal of a physical-to-electrical transducer.
Industrial measuring instruments as are described herein measure at least one physical quantity, such as the volumetric or mass flow rate of a fluid, the density, viscosity, pressure, or temperature of a fluid, the pressure difference between two media, or, quite generally, temperature, pressure, level, pH value, or gas concentration.
A conunon feature of such instruments is that a physical-to-electrical transducer delivers a signal that is converted by means of evaluation electronics to a measurement signal suitable for transmission and further processing.
Most of these instruments are designed as four-wire devices. Two of the four wires serve to supply power to the instrument, and the other two wires serve to transmit the meaasurement signal.
Less common are two-wire instruments, in which the two wires must be used both for the supply of power, which necessitates connecting an external DC voltage source to the two wires, and for the transmission of the measurement signal.
The measurement signal provided by the two-wire instrument is practically always an output current between 4 mA and 20 mA, with a particular current value within this range corresponding exactly to a measurement signal value. The current range below 4 mA is available for the supply of power to the evaluation electronics, which are also present in two-wire instruments.
Two-wire instruments require much less power than the aforementioned four-wire instruments; the power is composed of the aforementioned supply power for the evaluation electronics and the power corresponding to each current value.
Two-wire instruments are especially suited for use in hazardous areas. A difficulty is, however, that when such instruments are turned on, the small amount of supply power suffices for the evaluation electronics, but does not suffice to start the controlled current source.
Another disadvantage of two-wire instruments is that the supply power for the evaluation electronics varies with the measurement signal, i.e., in the case of a flow rate measurement with the flow rate. Therefore, a constant power supply for the evaluation electronics which is independent of the measurement signal, e.g., the flow rate, is desirable.
It is therefore an object of the invention to provide improved controlled current sources of two-wire instruments wherein the amount of power required at turn-on of the DC voltage source, which is greater than during normal operation, is made available, so that the controlled current source begins to operate automatically.
Furthermore, the supply power for the evaluation electronics is to be constant, i.e., independent of the measured value. In addition, a current source is to be provided which, besides attaining the above objects, is so designed that the so-called HART protocol can be transmitted over the two wires (HART is a registered trademark of the HART User Group).
To attain these objects, a first variant of the invention provides a controlled current source of a two-wire measuring instrument which generates a measurement signal in the form of an output current between 4 mA and 20 mA and which is controlled by a control signal derived from an output signal of a physical-to-electrical transducer, said controlled current source comprising:
a first current path,
which goes from a first terminal of a DC voltage source, to be connected from outside, via a diode, an emitter-collector path of a first bipolar transistor, a voltage regulator connected to ground, and a grounded current-sensing resistor to a second terminal of the DC voltage source;
a second current path,
which goes from the first terminal via the diode, the emitter-base path of the first bipolar transistor, a first resistor, a collector-emitter path of a second bipolar transistor, whose conductivity type is complementary to that of the first bipolar transistor, a second resistor, and the current-sensing resistor to the second terminal,
with the current in the first current path and the current in the second current path adding to the output current;
a feedback path for the output current
which goes from the second terminal through a feedback resistor to a noninverting input of an operational amplifier,
said noninverting input also receiving the control signal, and
an output of the operational amplifier being connected to the base of the second bipolar transistor; and
a transistor which renders the first bipolar transistor conductive after turn-on of the DC voltage source, said transistor
having its controlled current path connected in parallel with the emitter-collector path of the second bipolar transistor and
having its control electrode connected to a tap of a first RC section, containing a series resistor and a first capacitor,
the series resistor being connected to the collector of the first bipolar transistor, and the capacitor being connected to ground.
To attain the above objects, a second variant of the invention provides a controlled current source of a two-wire measuring instrument which generates a measurement signal in the form of an output current between 4 mA and 20 mA and which is controlled by a control signal derived from an output signal of a physical-to-electrical transducer, said controlled current source comprising:
a first current path,
which goes from a first terminal of a DC voltage source, to be connected from outside, via a first diode, an emitter-collector path of a first bipolar transistor, a voltage regulator connected to ground, and a grounded current-sensing resistor to a second terminal of the DC voltage source;
a second current path,
which goes from the first terminal via the first diode, the emitter-base path of the first bipolar transistor, a first resistor, a collector-emitter path of a second bipolar transistor, whose conductivity type is complementary to that of the first bipolar transistor, a second resistor, and the current-sensing resistor to the second terminal,
with the current in the first current path and the current in the second current path adding to the output current;
a feedback path for the output current
which goes from the second terminal through a feedback resistor to a noninverting input of an operational amplifier,
said noninverting input also receiving the control signal, andan output of the operational amplifier being connected to the base of the second bipolar transistor;
a first transistor, which renders the first bipolar transistor conductive after turn-on of the DC voltage source, and
to which a start signal is applied at a control electrode as a result of the turn-on; and
a second transistor, which turns the first transistor off after the end of the start phase, and
to which a stop signal is applied at a control electrode after the end of the start phase.
In a first preferred embodiment of the first variant of the invention, the first bipolar transistor is a pnp transistor, and the second bipolar transistor is an npn transistor.
In a second preferred embodiment of the first variant of the invention, which embodiment can be used together with the first embodiment, the transistor is a P-channel junction-gate field-effect transistor
whose drain terminal is connected to the collector of the second bipolar transistor and whose source terminal is connected to ground, and
whose gate terminal is the control electrode.
In a third preferred embodiment of the first variant of the invention, which can also be used together with the first or second embodiment, the control signal applied to the operational amplifier is a pulse-width-modulated signal which is generated from the output signal of the transducer by a microprocessor after turn-on of the DC voltage source, and which is smoothed.
In a first preferred embodiment of the second variant of the invention, the first bipolar transistor is a pnp transistor and the second bipolar transistor is an npn transistor.
In a second preferred embodiment of the second variant of the invention, which embodiment can be used together with the first embodiment of the second variant,
the first and second transistors are N-channel enhancement-mode insulated-gate field-effect transistors each having a drain, a source, and a gate terminal;
the gate terminal of the first transistor is connected through a fourth resistor in series with a third resistor to the base of the first bipolar transistor;
the drain terminal of the first transistor is connected through a fifth resistor in series with the fourth resistor to the base of the first bipolar transistor;
the source terminal of the first transistor is connected to ground;
the drain terminal of the second transistor is connected to the gate terminal of the first transistor;
the source terminal of the second transistor is connected to ground; and
the gate terminal of the second transistor is connected to an output of a microprocessor which provides the stop signal.
In a third preferred embodiment of the second variant of the invention, which embodiment can also be used together with the second and/or third embodiments of this variant, the control signal applied to the operational amplifier is a pulse-width-modulated signal which is generated from the output signal of the transducer by a microprocessor and which is smoothed,
In a fourth preferred embodiment of the second variant of the invention, which can also be used together with the first to the third embodiments of this variant,
an output of a HART protocol transmitting stage is capacitively coupled to the noninverting input of the operational amplifier, this HART protocol transmitting stage serving to modulate a first HART protocol signal to be sent out of the two-wire measuring instrument upon the output current;
an input of a HART protocol receiving stage is capacitively coupled to the emitter of the first bipolar transistor, this HART protocol receiving stage serving to demodulate a HART protocol signal modulated upon the output current outside of, and to be received by, the two-wire measuring instrument; and
the collector of the first bipolar transistor is coupled to a first terminal of a capacitor having its second terminal connected to ground.
According to a development of the fourth embodiment of the second variant of the invention, a second diode is inserted between the collector of the first bipolar transistor and the voltage regulator.